BASIC Compiler

6 Example: BASIC Compiler Program ®le for this chapter: basic The BASIC programming language was designed by John Kemeny and Thomas Kurtz in the late 1960s. (The name is an acronym for Beginner's All-purpose Symbolic Instruction Code.) It was ®rst implemented on a large, central computer facility at Dartmouth; the designers' goal was to have a language that all students could use for simple problems, in contrast to the arcane programming languages used by most experts at that time. A decade later, when the microcomputer was invented, BASIC took on a new importance. Kemeny and Kurtz designed a simple language for the sake of the users, but that simplicity also made the language easy for the computer! Every programming language requires a computer program to translate it into instructions that the computer can carry out. For example, the Logo programs you write are translated by a Logo interpreter. But Logo is a relatively complex language, and a Logo interpreter is a pretty big program. The ®rst microcomputers had only a few thousand bytes of memory. (Today's home computers, by contrast, have several million bytes.) Those early personal computers couldn't handle Logo, but it was possible to write a BASIC interpreter that would ®t them. As a result, BASIC became the near-universal language for amateur computer enthusiasts in the late 1970s and early 1980s. Today's personal computers come with translators for a wide variety of programming languages, and also with software packages that enable many people to accomplish their computing tasks without writing programs of their own at all. BASIC is much less widely used today, although it has served as the core for Microsoft's ªVisual Basicº language. In this chapter, I want to show how Logo's define command can be used in a program-writing program. My program will translate BASIC programs into Logo programs. I chose BASIC for the same reason the early microcomputers used it: It's a small language and the translator is relatively easy to write. (Kemeny and Kurtz, the designers of BASIC, have criticized the microcomputer implementations as too simple 81 and as unfaithful to their original goals. My implementation will share that defect, to make the project easier. Don't use this version as a basis on which to judge the language! For that you should investigate True Basic, the version that Kemeny and Kurtz wrote themselves for personal computers.) Here's a typical short BASIC program: 10 print "Table of Squares" 20 print 30 print "How many values would you like?" 40 input num 50 for i=1 to num 60 print i, i*i 70 next i 80 end And here's what happens when we run it: Table of Squares How many values would you like? 5 1 1 2 4 3 9 4 16 5 25 A Short Course in BASIC Each line in the sample BASIC program begins with a line number. These numbers are used for program editing. Instead of the modern screen editors with which you're familiar, the early versions of BASIC had a very primitive editing facility; you could replace a line by typing a new line with the same number. There was no way to replace less than an entire line. To delete a line completely, you'd enter a line containing just the number. The reason the line numbers in this program are multiples of ten is to leave room for inserting new lines. For example, I could say 75 print "Have a nice day." to insert a new line between lines 70 and 80. (By the way, the earliest versions of Logo used a similar line numbering system, except that each Logo procedure was separately 82 Chapter 6 Example: BASIC Compiler numbered. The editing technique isn't really part of the language design; early systems used ªline editorsº because they had typewriter-like paper terminals instead of today's display screens. I'm using a line editor in this project because it's easy to implement!) The BASIC language consists of one or two dozen commands, depending on the version used. My BASIC dialect understands only these ten commands: LET variable = value PRINT values INPUT variables FOR variable = value TO value NEXT variable IF value THEN command GOTO linenumber GOSUB linenumber RETURN END Unlike Logo procedure calls, which consist of the procedure name followed by inputs in a uniform format, each BASIC command has its own format, sometimes including internal separators such as the equal sign and the word to in the for command format, or the word then in the if command format. In some versions of BASIC, including this one, a single line can contain more than one command, if the commands are separated with colons. Thus the same program shown earlier could also be written this way: 10 print "Table of Squares":print 30 print "How many values would you like?":input num 50 for i=1 to num : print i, i*i : next i 80 end The let command assigns a value to a variable, like Logo's make procedure. Unlike Logo, BASIC does not have the rule that all inputs are evaluated before applying the command. In particular, the word after let must be the name of the variable, not an expression whose value is the name. Therefore the name is not quoted. Also, a variable can't have the same name as a procedure, so there is no need for anything like Logo's use of the colon to indicate a variable value. (This restricted version of BASIC doesn't have named procedures at all, like some early microcomputer versions.) make "x :y + 3 (Logo) let x = y + 3 (BASIC) A Short Course in BASIC 83 In my subset of BASIC, the value of a variable must be a number. More complete BASIC dialects include string variables (like words in Logo) and arrays (like Logo's arrays). The value to be assigned to a variable can be computed using an arithmetic expression made up of variables, numbers, the arithmetic operators +, -, *, and /, and parentheses for grouping. The print command is similar to Logo's print procedure in that it prints a line on the screen. That line can include any number of values. Here is an example print command: print "x = "; x, "y = "; y, "sum = "; x+y In this example two kinds of values are printed: arithmetic values (as in the let command) and strings. A string is any sequence of characters surrounded by quotation marks. Notice that the values in this example are separated by punctuation marks, either commas or semicolons. When a semicolon is used, the two values are printed right next to each other, with no space between them. (That's why each of the strings in this example ends with a space.) When a comma is used, BASIC prints a tab character between the two values, so that values on different lines will line up to form columns. (Look again at the table of squares example at the beginning of this chapter.) The input command is the opposite of print; it reads values from the keyboard and assigns them to variables. There is nothing in Logo exactly like input. Instead, Logo has operations readword and readlist that output the contents of a line; those values can be assigned to variables using make or can be used in some other way. The Logo approach is more ¯exible, but the early versions of BASIC didn't have anything like Logo's operations. The input command will also accept a string in quotation marks before its list of variables; that string is printed as a prompt before BASIC reads from the keyboard. (BASIC does not start a new line after printing the prompt, so the effect is like Logo's type command rather than like print.) Here's an example: input "Please enter x and y: " x,y The user can type the values for x and y on the same line, separated by spaces, or on separate lines. BASIC keeps reading lines until it has collected enough numbers for the listed variables. Notice that the variable names in the input command must be separated by commas, not by semicolons. The for and next commands work together to provide a numeric iteration capability like Berkeley Logo's for procedure. The for command format includes a 84 Chapter 6 Example: BASIC Compiler variable name, a starting value, and an ending value. (The step value is always 1.) The named variable is given the speci®ed starting value. If that value is less than the ending value, then all of the commands between the for command and the matching next command (the one with the same named variable) are carried out. Then the variable is increased by 1, and the process continues until the ending value is reached. For and next pairs with different variables can be nested: 10 input "Input size: " num 20 for i = 1 to num 30 for j = i to num 40 print i;" ";j 50 next j:next i 60 end Input size: 4 1 1 1 2 1 3 1 4 2 2 2 3 2 4 3 3 3 4 4 4 Notice that the next j must come before the next i so that the for/next pairs are properly nested. The if command allows conditional execution, much like Logo's if command, but with a different notation. Instead of taking an instruction list as an input, BASIC's if uses the keyword then to introduce a single conditional command. (If you want to make more than one command conditional, you must combine if with goto, described next.) The value that controls the if must be computed using one of the operators =, <, or > for numeric comparison.* * Notice that the equal sign has two meanings in BASIC.

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